A loudspeaker typically has a constant radiation characteristic. The radiation characteristic of loudspeakers incorporated into sound systems can be changed to a limited degree by suitable placement of the loudspeakers in the auditorium and/or with the help of additional mechanical devices, such as acoustic reflectors or acoustic lenses. Sound systems in automobiles are subject to particularly stringent requirements since neither can the location for installation of the loudspeakers in general be arbitrarily selected, nor is there enough available space for installing additional mechanical devices. Moreover, since the space into which the sound is radiated is typically relatively small, the channels of, for example, stereo sound can frequently not be adequately separated, in particular for more than one listener.
JP 07-046 698 A discloses a circuit with left and right audio signal channels, with each channel having an adder unit which admixes to the original audio channel signal a phase-shifted portion of the sound signal from the opposite channel with a modified amplitude (cross-coupling).
JP 06-315 198 A discloses a similar circuit with an additional control unit for adjusting the amplifiers and phase shifters in the cross-coupling branches. The reference shows loudspeakers connected to the outputs. Hence, JP 06-315 198 A shows an arrangement where respective sums are formed from input signals that have differently weighted amplitudes and phases.
JP 04-337 999 A discloses a device with several loudspeakers controlled by phase shifters, wherein the radiation direction of the loudspeakers can be altered using the phase shifters.
U.S. Pat. No. 5,305,386 A, in particular FIG. 4 (2), discloses a circuit with cross-coupled branches that include attenuation units and phase shifters. This circuit forms part of an overall circuitry for expanding and controlling tonal responses (“sound fields”).
WO 97 30 566 A1 is also directed to imaging virtual sound sources at predetermined locations by taking also into consideration the radiation angle enclosed by the loudspeakers (e.g., 10 degrees). For this purpose, filter circuits H1(z) and H2(z) which introduce phase shifts, are provided in both the main branches and the cross-coupled branches.
JP 02-241 296 A discloses an arrangement with four loudspeakers which are placed in an automobile in front of the driver and the passenger. The loudspeakers operate so that the driver as well as the passenger can perceive a usable stereo tonal response. However, this arrangement includes neither phase shifters nor cross couplers.
It would therefore be desirable to provide a sound system wherein the radiation characteristic can be changed without requiring adjustments in the position of the loudspeakers and without employing additional mechanical devices.